Reaction bonded silicon nitride ("RBSN") -based ceramic materials are currently viewed as useful materials for high temperature applications due to their superior thermal and mechanical properties. For example, containers used in the production of silicon billets are typically made from fused silica. However, upon cooling, the fused silica often devitrifies and forms pockets of cristobalite. The cooling of cristobalite, in turn, induces a volume expansion which causes the container to crack. Ceramics less susceptible to thermal expansion have been studied as potential replacement materials for silica in these containers. One ceramic, silicon nitride, has been targeted as one particularly suitable replacement material. RBSN also possesses chemical compatability in applications that would react with carbides and oxides, a favorably low thermal expansion coefficient, a high surface tension, and so is suitable for applications involving silicon handling and other refractory applications.
In many applications, silicon nitride ceramics are produced by the reaction bonded method. This method entails forming a green body from a fine (i.e., less than 20 micron) silicon powder and exposing the body to a nitrogen atmosphere at a maximum temperature of about 1450.degree. C., thereby nitriding the elemental silicon into reaction bonded silicon nitride ("RBSN"). See, e.g., U.S. Pat. No. 4,521,358. This approach typically produces a near net shape ceramic useful in refractory applications.
However, the art has typically experienced problems in developing RBSN technology. For example, because fine silicon powders tend to be very expensive, the cost of producing an RBSN is relatively high. In addition, when fine silicon powders are produced by milling coarse silicon powders, the milling often introduces contamination into the powders, thereby rendering the fine silicon powder less pure. Moreover, the typical silicon powder green body produced by conventional methods possesses low green strength, thereby making large shape fabrication difficult. As a result, handling these low strength green bodies is often problematic. Lastly, there are many ceramic applications which would benefit from utilizing a ceramic having free silicon embedded therein. However, conventional RBSN technology does not teach how to make RBSN's having free silicon embedded therein which is shielded from environmental attack, and in particular, an RBSN having free silicon embedded therein usable above the melting point of silicon (1410.degree. C.).
RBSN processing has also encountered difficulties. The nitriding reaction discussed above is an exothermic reaction producing about 733 KJ/mol of heat. If the rate of nitridation is uncontrolled, then the temperature increase resulting from this reaction is typically significant enough to at least partially melt the elemental silicon, thereby causing meltout. Three process modifications to mitigate the exotherm problem have been suggested. One modification limits the rate at which nitrogen gas is introduced into the furnace, thereby reducing the amount of nitrogen available for reaction. Another modification adds an inert gas such as argon or helium into the nitrogen stream, thereby diluting the nitrogen atmosphere. A third modification adds a nonreactive material such as silicon nitride, silicon carbide or boron nitride to the green body. This approach not only reduces the reaction rate, it also provides a thermal sink. Although each of these suggested means for controlling the exothermic reaction has advantages, each also has its drawbacks. In particular, each requires strict control over the reaction, thereby raising the cost of RBSN production and narrowing its applicability. For example, adding a non-reactive filler material to the green body decreases the extent of reaction and thus the final density, and also significantly increases the raw material cost. In addition, the use of silicon carbide as a filler may unacceptably raise the thermal expansion coefficient of the RBSN. The use of a silicon nitride seed typically consists of a high surface area particle which has high reactivity.
Thus, it is an object of the present invention to provide a silicon nitride ceramic which is inexpensive, preferably has high purity, more preferably can be formed into large shapes, and most preferably contains embedded free silicon which is shielded from environmental attack and is functional above the melting point of silicon. It is a further object of the present invention to provide a method of making reaction bonded silicon nitride without requiring strict process control in the nitridation reaction.